The present invention generally relates to a method and apparatus for induction heat treating and restraint quenching of structural members generally having a web and at least one flange. The object of this heat treatment is to produce certain physical properties which are required in the design of truck side rails, as a means of increasing strength of a given side rail section such that the payload capability of a truck is increased while holding down the weight thus effecting a more energy efficient vehicle. Truck side rails are generally long structural channel members generally 18 to 40 feet in length, 0.210 to 0.375 inch thick. Due to the use of a carbon steel, a rapid effective quench is necessary to obtain the required minimum physical properties and microstructure. As the truck side rail is a long, thin section member, generally cold formed into its channel shape from a flat strip, considerable residual stresses are present due to the forming operation. During the heating and rapid quenching of these thin section members, considerable distortion occurs due to the interaction of the residual stresses and the severe thermal action of the quench; distortion is evident in the member as bow, twist, and camber, as well as growth experienced along the length and width of the side rail due to the material grain size enlargement and mechanical working as a result of the heat treat operation.
The present invention more specifically relates to a method and apparatus for induction heat treating and restraint quenching structural members such that the total system represents a more economical investment than do existing conventional methods of restraint quenching, in particular, restraint quenching truck side rails. These other techniques for restraint quenching truck side rails are taught in U.S. Pat. Nos. 3,252,695 and 3,294,597 in which are employed large movable die quench sections constructed with a vast network of quench fluid passages to simultaneously allow quench fluid passage and provide restraint, such that the overall effect is akin to a large forming press complete with dies, representing a sizeable investment. The method of restraint taught in conventional methods must be elaborate as the entire member is quenched at the same time, setting up significant thermal gradients along the member length, greatly increasing its propensity to distort. The present invention does not require the large quench press and its attendant special die sections as relatively short sections of the rail are heated in stages to austenitizing temperature and then immediately restraint quenched on a continuing basis as the member travels longitudinally through the system, necessitating only a relatively short restraint quench system which, in the present invention, employs special restraint rolls.
It is known generally to produce induction heated, quenched and tempered, high strength steels for tubular products but not necessarily for structural members, more specifically channel members. One method of making these is described by Tom Allan in a paper entitled "Production of High Strength Quenched and Tempered ERW Tubular Products Utilizing Induction Heating Equipment," 13th Mechanical Working and Steel Processing Conference, Jan. 20, 1971, Pittsburgh, Pa.
Generally others have taught various techniques for rapid heat treating of steel and quenching. Examples of this are R. A. Grange, "The Rapid Heat Treatment of Steel", Metallurgical Transactions, Vol. 2, January 1971, p. 65-78. More generally, the induction and flame hardening of metal is known as shown in the Metals Handbook. Vol. 2, 8th Edition, "Induction and Flame Hardening", pages 167-202. The particular type of hardening taught in this instance is known as "through hardening". Other developments of bar quenching fixtures are taught in U.S. Pat. No. 3,738,629. Various inductor coil designs are taught in U.S. Pat. No. 3,699,302 and in the Metals Handbook, Vol. 2, page 171.
None of these techniques have to date achieved significant commercial use because of the serious problems of bowing, twisting and cambering associated therewith or the significant capital investment necessary to achieve the high quality, high strength product called for by user specifications.
A more recent improvement in production heat treatment is U.S. Pat. No. 4,142,923 which teaches the use of inductor coil heating, liquid quenching under restraint and tempering under restraint generally. While this system reduces capital investment, problems with ability to control localized bowing and dislocation of holes subsequently punched in the structural member caused by cambering in the heat treat process have not been solved sufficiently to achieve the high quality high strength members on a continuous production basis.
Thus, it is apparent that improvements in the art are needed to provide a method and apparatus for heat treating structural members on a production basis while maintaining the distortions to within the user specifications for the product structural members.